Cathode-ray tube for the reproduction of colour television images



March 4, 1958 J. DE GIER CATHODE-RAY TUBE FOR THE REPRODUCTION OF COLOURTELEVISION ISAAGES Filed April 2'7, 19

I ,I f l Bandwidth Bandwidth Bandwidth A A+A A A INVENTOR IJOHANNES DEGIER rates CATHODlE-RAY TUBE FOR THE REPRODUCTIO F COLOUR TELEVISIONllVIAGES Johannes De Gier, Eindhoven, Netherlands, assignor, by mesneassignments, to North American Philips Company, Inc., New York, 'N. Y.,a corporation of Dela- The invention relates to cathode-ray tubes andapparatus for the reproduction of coloured television images in whichthe tube contains at least two electron guns.

For the reproduction of coloured television images by means of a singlecathode-ray tube widely different tube constructions have beensuggested. Inter alia, a cathoderay tube has been described having threeelectron guns by means of which three concentrated electron beams can bedirected to an image screen containing three luminescent substancesemitting light of different colours, which substances are arrangeduniformly in discrete surfaces and, .in the operation of the tube, caneach be excited by a beam from a single electron gun. In order to buildup the image, a predetermined signal is supplied to each of the guns. Ithas been described that it is not necessary to supply signals of thesame bandwidth to the three guns, for, in order to ensure the productionof a satisfactory coloured image, it is sufiicient that one of thecolouredimages has a high definition and consequently it sufiices when asignal of large bandwidth is supplied to the electron gun for thiscolour. Thus, this gun determines the definition of the image. In mostsystems in present use the three primary colours chosen are red, greenand blue, and for visual observation it has proved to be of advantage todetermine the definition by means of the green image. However, forspecial purposes the definition may be determined by any of the otherprimary colours. The blue image will, for example, be used, when use ismade ofan image amplifier connected after the colour tube, whichamplifier has a maximum sensitivity in the blue region.

Although it has not been described, it will be obvious that what hasbeen written hereinbefore with respect to the bandwidth also applies tothe case where two or more than three electron guns and a correspondingnumber of primary colours are used.

The use of the difierent bandwidth was discovered in the search for acomposite television signal which is suitable for wireless transmissionand does not have an excessively large over all bandwidth, preferably abandwidth not exceeding that of a signal .of the kind used inblack-and-white television.

In a cathode-ray tube in accordance with the invention the constructionof the electron guns has been made to conform with the insight gainedinto the required bandwidth, with resulting advantages which will bedescribed more fully hereinafter.

A cathode-ray tube in accordance with the invention for the reproductionof coloured television images comprises an image screen provided with atleast two luminescent materials, which, on electron bombardment, emitlight of difierent colours and with difierent efficiency and arearranged in discrete uniformly distributed surfaces, and at least twoelectron guns, which each comprise at least a cathode, a control gridand an anode and which generate electron beams which each pass through aprincipal or main focus'sing lens having the same image atent O2,825,847 ."Patented Mar. 4, I958 ratio and each co-operate withluminescent material emitting light of one colour only, thecontrolelectrode of the gun which cooperates with a luminescent material ofhigher efliciency having a signal supplied to it which has a greaterbandwidth than the signal-supplied to the control electrode of thegun'co-operating-with a luminescent material of lower efficiency. Thegeometry of the tube is such that the spot size of the gun co-operatingwith luminescent material of higher efficiency at a predeterminedcurrent strength and with maximum focusing by the principal lens issmaller than that of the gun cooperating with a luminescent material oflower efiiciency, when compared under conditions of the same currentstrength and with maximum focusing.

The term image ratio of the principal lens as used herein is to beunderstood to mean the ratiobetween'the distance by which the infinitelythin lens, which with respect to its efiect may replace the principallens, is spaced away from the image screen and the distance by which itis spaced away from the control electrode. This image ratiois analogousto the image ratio of an optical lens. In actual fact, the object, animage of whichfis produced electron-optically, is the cross-overproduced in close proximity to the control electrode. In practice it isimmaterial whether use is made of the distance between the principallens and the cross-over or of the dis .tance between the principal lensand the control electrode. However, the latter distance can more readilybe measured in the tube.

Since the image ratios of the principal lenses are equal for the variouselectron guns and the focusing is adjusted to have its maximum value, alarger beam spot on the image screen means that the cross-over also islarger, for the cross-over is the object of whichan image is produced bythe principal lens. A larger cross-over means a smaller specific cathodeload, i. e. the current persq. cm. of the operative cathode surface.From this it follows that in a cathode-ray tube in accordance with theinvention, the specific cathode loads of all the electron gunsapproximate to one another as closely as possible. This can be explainedas follows:

The luminescent material with which the gun for the greater bandwidthco-operates, requires acertain current strength to produce apredetermined amount of light. In order to ensure that the luminescentmaterial of smaller efiiciency, which consequently is excited by theelectron beam of a gun to which a signal of smaller bandwidth issupplied, produces an amount of light which matches the first-mentionedamount of light, a greater current strength in the electron beam isnecessary. If the size of the beam spot on this luminescent materialwere made equal to that on the luminescent material of higherefiiciency, there would be, for a cross-over of equal surface area, agreater current strength, and this means that the specific cathode loadof the gun producing this cross over is greater. Since the specificcathode load determines the lifetime of the cathode, the life of thelastmentioned cathode would be shorter than that of the first-mentionedcathode. It is obvious that this is very inconvenient, since the gunsand consequently the cathodes are arranged in one tube and hence failureof on'e cathode necessitates replacement of the tube or viewing picturesin unnatural colours henceforth- However, as has been describedhereinbefor'e, it is not necessary that the definition of all the imagesbe equal and hence when the higher definition is produced in the screenof the highest efiiciency, the spot on the screen material of smallerefficiency can be made larger. As has been explained hereinbefore, thisresults in a smaller specific cathode load in the gun which co-operateswith the ice luminescent material of lower efi'ic'iency. Although insome cases the specific cathode loads cannot be made exactly equal inthis manner, the longest possible lifetime will thus be obtained for allthe cathodes.

Since the size of the cross-over is also dependent upon the value of thecurrent in the electron beam, any comparison of beam spot sizes mustobviously be made with the same current strength. In the operation of atube in accordance with the invention, the current in the electron beamproduced by the gun which co-operates with the luminescent material oflower efliciency will exceed the current in the beam of the gun whichco-operates with the luminescent material of higher efiiciency.

At a predetermined value of the current strength, a larger cross-overand consequently a larger spot can, for example, be obtained by a largeraperture of passage in the control electrode and by a larger spacingbetween the cathode and the control electrode. Consequently, variationsin the geometry of the electron gun permit of obtaining a predeterminedsize of the cross-over and consequently of the spot on the image screen,at a given current strength.

A larger aperture of passage of a control electrode results in thereduction of the so-called cut-01f voltage, i. e. the voltage of thecontrol electrode at which the electron beam current is reduced to zero.By the choice of the cathode-control electrode spacing, the cut-offvoltages can be made substantially equal for the various guns. Thus, acomplicated circuit arrangement is avoided, which would be required, ifuse were made of guns having different cut- 05 voltages.

In may cases it will be necessary to arrange in front of the imagescreen a mask electrode, which ensures that electrons from one gun canexciteluminescent material only which emits light of one colour. Ifrequired, the electron beams may be additionally concentrated in knownmanner between the mask and the image screen. In this embodiment, thespot size must be measured at the mask.

The electron beams produced by the guns can be passed through a commonprincipal lens. By ensuring that in all the guns the cathode is spacedaway from the principal lens by the same distance, it can be ensuredthat the image ratio is the same for all the guns.

The invention will now be described more fully with reference to apractical embodiment, given by way of example, of a cathode-ray tube forvisual observation, in which use is made of 3 luminescent materials,which emit green, red and blue light respectively. As luminescentmaterials use was made for the green image of willemite, for the redimage of zinc beryllium silicate activated with manganese, and for theblue image of calcium magnesium silicate activated with titanium. Of thesaid three substances, the willemite has the highest efiiciency. Sincethe tube is to be used for visual observation, it is of advantage todetermine the definition by the green image. Consequently, the spot onthe material luminescent in green must be smallest. Since, in measuring,a raster was traced out, the line width was measured instead of the spotsize, which amounts to the same thing.

With a line width of 0.033 mm., a well-defined green image was obtainedwith a current strength of 90 micro- .amperes. The specific cathode loadof the corresponding electron gun was 0.146 amp/sq. cm.

In order to obtain an image having a brightness matching the brightnessof the green image, with the same linewidth, on the material luminescentin red a current strength of 200 micro-amps. was required. The specificcathode load of the corresponding gun in this case would be 0.28 amp/sq.cm. It was found that the linewidth on the material luminescent in redcould be enlarged to 0.065 mm. without the image produced by thecombination of the images being seriously afiected. However,- to obtainsatisfactory brightness, the current strength has to The specificcathode load of the h was r d c d to 1.

remain 200 micro-amps. gun for the red image,

' the same current strength of amp/sq. cm., only slightly larger thanthat of the green gun.

With a same linewidth as measured on the screen luminescent in green, onthe material luminescent in blue a current strength in the electron beamof 120 microamps. was found for an image having abrightness matchingthat of the green and red images. The specific cathode load of thecorresponding gun in this case was 0.174 amp/sq. cm. It was found thatthe line width on the material luminescent in blue could be enlarged to0.065 mm. without the combination picture being aifected. Withmicro-amps, the specific cathode load was reduced to 0.126 amp/sq. cm.

The diameter of the area of passage of the control electrode was 0.5 mm.in the electron gun for the green image and 0.6 mm. in the two otherelectron guns.

The cut-oif voltages were equal for all the three guns, i. e. 60 volts.In the electron gun for the green image the spacing between the controlelectrode and the cathode was 0.2 mm. and in the two other guns 0.25 mm.The thickness of the control electrode and the spacings between the saidelectrode and the screen were equal in all the electrode systems.

The invention will now. be described more fully with reference to theaccompanying drawing, of which the single view is a partly schematic,partly cross-sectional view of cathode-ray tube apparatus according tothe invention.

In the drawing, the cathode-ray tube shown consists of a cylindricalneck portion 1 and a mating conical part 2 closed off by a transparentwindow 3. This window supports a luminescent screen 4 containing threeluminescent materials,- luminescing in three primary colors, arranged ina repetitive pattern. In the neck 1 of the tube, three electron guns arearranged, in which the two outer electron guns are identical. .These twoguns comprise a cathode 5, a diaphragm 6 acting as a control grid, anaccelerating or first anode 7 and a final anode 8. Due to thecooperation of the electrodes 5, 6 and .7, which serve as an immersionor first lens, a cross-over is produced approximately in the aperture ofthe control grid 6. This node is projected onto the luminescent screen 4by means of a second electron lens formed by a focussing coil 9, whichlens is called the main or principal lens. This main lens is as may beseen common to the cathode-rays produced by the three guns. Apart fromfocussing each electron beam, this main lens at the same time convergesthe three beams to a'common point 15 of the image screen 4. Between themain lens formed by the coil 9 and the screen the three electron beamsare deflected with the aid of defleeting coils 13 and 14.

As will be seen from'the' drawing, the central gun is of a constructionwhich is for the greater part similar to the construction of the twoouter guns. However, in the central gun, the aperture or passage in thecontrol grid 6 is smaller than the corresponding apertures in thecontrol grids of the two outer guns. Moreover, the distance between thecathode and the control-grid is smaller for the central gun than for thetwo outer guns. As shown in the drawing, three signal sources areprovided for applying signals of bandwidth A and a signal of largerbandwidth A-l-AA to the control grids 6 of the three guns for modulatingtheir beam current. In accordance with what is described above, thesignal with the greater bandwidth A+AA is applied to the central gun.This central gun, which produces a smaller beam spot, co-operates withthe luminescent material of higher efficiency on the screen 4.

What is claimed is:

1. A cathode-ray tube comprising an image screen containing pluralluminescent materials possessing higher and lower efiiciencies andluminescing in plural colors to form a composite color image, and aplurality of electron guns each producing an electron beam for excitingone of Sa d luminescent materials, said gun exciting the higheraeaaswefficiency luminescent material producing a smaller beam spot size inthe vicinity of the screen than that of the other guns, when comparedunder conditions of the same beam current and the same beam focnssing;

2. A tube as set forth in claim 1 wherein each of the guns includes acathode and an apertured control grid, the control grid of the gunexciting the higher-emciency luminescent material having a smalleraperture than that of the control grids of the other guns.

3. A tube as set forth in claim 1 wherein each of the guns includes acathode and an apertured control grid, the spacing between the cathodeand the control grid of the gun exciting the higher-efficiencyluminescent material being smaller than that of the other guns.

4. Cathode-ray tube apparatus comprising: a cathoderay tube having animage screen containing plural luminescent materials possessing higherand lower efficiencies and luminescing in plural colors to form acomposite color image, and a plurality of electron guns each includingcathode and apertured control grid electrodes and each producing anelectron beam for exciting one of said luminescent materials; and aprincipal lens associated with the guns and having the same image ratiofor each of the guns for focussing the beams onto the image screen; saidgun exciting said higher-efliciency luminescent material producing asmaller beam spot size in the vicinity of the screen than that of theother guns, when compared under conditions of equal beam current andmaximum focussing by the principal lens.

5. Apparatus as set forth in claim 4 wherein, in the gun exciting thehigher-efiiciency luminescent material, the aperture in the control gridis smaller than, and the cathode-control grid spacing is smaller than,the corresponding values for the other guns.

6. Cathode-ray tube apparatus comprising: a cathoderay tube having animage screen containing plural luminescent materials possessing higherand lower eficiencies and luminescing in plural colors to form acomposite color image, and a plurality of electron guns each includingcathode and apertured control grid electrodes and each producing anelectron beam for exciting one of said luminescent materials; aprincipal lens associated with the guns and having the same image ratiofor each of the guns for focussing the beams onto the image screen; saidgun exciting said higher-efiiciency luminescent material producing asmaller beam spot size in the vicinity of the screen than that of theother guns, when compared under conditions of equal beam current andmaximum focussing by the principal lens; plural signal sourcesgenerating signals of larger and smaller bandwidth; and means forapplying the larger bandwidth signal to the gun exciting thehigherefliciency material and the signals of smaller bandwidth to theother guns.

7. Apparatus as set forth in claim 6 wherein, in the gun exciting thehigher-efficiency luminescent material, the aperture in the control gridis smaller than, and the cathode-control grid spacing is smaller than,the corresponding values for the other guns, said values being such thatthe beams of all the guns are cut-ofi by the same voltage.

8. Apparatus as set forth in claim 6, wherein a common principal lens isprovided for all the guns, and the cathode-principal lens spacing is thesame for all the guns.

References Cited in the file of this patent UNITED STATES PATENTS

